In order to protect the copyrights of content providers, information recording media on which video content is recorded require a mechanism to prevent the illegitimate reproduction or use of the content. For this reason, in a technique applied to DVDs, an ID number unique to the disc is recorded and the ID number is used to encrypt the content data.
As a method of recording the unique ID number of a disc, a technique has been proposed (see Patent Document 1) in which a barcode formation area is provided on the inner circumferential side of a reproduce-only (ROM) disc, and part of the reflective film is selectively removed by laser trimming from the finished ROM disc to form a barcode. This barcode formation area is known as the BCA (Burst Cutting Area). Proposed techniques for preventing illegitimate copying of the BCA include the embedding of secret information in the area by changing the locations of the marks constituting the BCA within their tolerance range (see Patent Document 2), and disabling BCA recording by producing degenerative changes in the disc (see Patent Document 3).
In recent years, systems have been put into practice in which content is delivered via a network and users record the received content on recordable optical discs. The recordable optical discs used in these systems also have an ID number unique to each disc recorded by use of the BCA, and the received content data are encrypted by use of the unique ID number of the disc before being recorded on the disc. After recording, even if the content data written on the disc are copied faithfully onto another disc, the data will not be decrypted correctly if the other disc has a different ID number, so the reproducing of illegitimately copied discs can be prevented.
However, if BCA recording is too easy, there remains a risk that the BCA may be copied. If the BCA is copied and the content data are copied bit-by-bit, the disc can be reproduced correctly by using the ID number recorded in the copied BCA. Accordingly, if the BCA can be easily copied by use of, for example, an optical head used for recording user data, there is a high risk that the manufacture of pirated copies will also be easy. Since the illegitimately copied pirated discs are clone discs carrying exactly the same information as the original disc, they cannot be distinguished from genuine discs with legitimately written information, and copyright protection technology cannot keep them from being reproduced.
One conceivable way to prevent the manufacture of pirated copies is to make BCA recording more difficult by the use of special equipment. The special equipment uses high-power laser trimming to effect a partial removal of the recording film, as on a reproduce-only optical disc. However, even if the identifying information is recorded on the BCA by partially removing the recording film in this way, there are recordable optical discs using reflectance reducing recording films, the reflectance of which is reduced when a mark is recorded in the BCA, so recorded marks also show reduced reflectance. FIG. 6 shows the reflectance distribution of the BCA on a recordable optical disc using a reflectance reducing recording film. The reflectance is reduced both where marks are recorded and where the recording film is removed, so depending on the reproduced signal level, recorded marks may be indistinguishable from places from which the recording film has been removed. This allows the use of recorded marks to mimic the BCA pattern formed by removing the recording film, making it difficult to prevent the manufacture of pirated copies.